The present invention relates to combined water meters having a main meter for measuring larger flows, an automatically acting switch valve arranged following the main meter, a secondary meter in a secondary line or secondary channel for measuring smaller flows, and an outlet from the secondary line or channel which debouches laterally into the housing of the switch valve.
Combined water meters and automatically acting switch valves which serve for switching between a main meter and a secondary meter were used in the nineteenth century. They include a valve disk including a surface which is larger than the surface of the sealing seat of the main water stream. As a result, the differential water pressure acts on a smaller surface in a blocked passage and on a larger surface in an open passage in order to abruptly open the valve disk. In order to counterbalance for the differential water pressure when the valve disk is closed, suitable means are employed, for instance, gravity, spring force, etc. It is desired that the valve disk open and close in each case as suddenly as possible so that no errors in measurement will occur.
For example, in Federal Republic of Germany C-92 631, the valve disk is seated within a guide cylinder which is arranged after the valve seat. The annular slot between the rim of the valve disk and the wall of the cylinder is kept as narrow as possible. This construction, however, cannot produce the desired sudden lift off of the valve disk because after the slightest lifting of the valve disk from its sealing seat, pressure equalization occurs which reduces the difference in pressure. The equalization is produced via the unavoidable annular slot, which, with constant flow, causes the valve disk to stop within the region of the cylinder and thus causes a too slight flow through the main meter. Accordingly, large measurement errors occur with this construction, upon both the opening and the closing of the valve.
Federal Republic of Germany C-10 77 884 which was published in 1960, discloses an improved switch valve for combined meters. It operates with a swing flap which is equipped with a protruding rim which engages into an annular space into which the secondary stream coming from the secondary meter is introduced. The swing flap is held in a closed position by a weight which is arranged so that a reduction in the closing force is obtained upon the opening of the flap. This change in the closing force in combination with the pushing action of the secondary stream on the rim of the flap accelerates the opening process but decelerates the closing process. As a whole, however, this reduces the errors in measurement during the switching. These switch valves can be mounted only in a well defined position. Furthermore, they are extremely large and heavy, making them completely unsuitable for the so called short structural length preferred today.
In about 1980, switch valves were defined for combined water meters having a valve disk which moves linearly against a spring as described in Federal Republic of Germany C-30 34 056. The valves have a double sealing seat and require a fine control hole for their operation. The valve disk also has a widened rim which extends into an annular space into which the secondary stream enters. In this way, the secondary stream pushes in the opening direction, and this push must be counteracted by a suitably strengthened closure spring. These switch valves produce an error in measurement, even though a slight one, in the switching region. Dirt, rust or lime in the water can interfere with the dependable operation of the switch valve and increase the errors in measurement.
All switch valves having a closure spring have one problem in common. The restoring force of the spring increases linearly with an increasing path of opening of the valve disk. The ideal, however, would be a constant or even a decreasing restoring force, as is present in the above mentioned weight loaded flap valves. Spring loaded switch valves must therefore be provided with a spring which is as long as possible in order to keep the increase in force slight. This, however, runs counter to the so-called short construction length which is preferred today.
Federal Republic of Germany A-39 29 391 discloses a switch valve for combined water meters which uses a permanent magnet in order to exert an additional holding force on the disk in its closed position. This enables the restoring spring to be made correspondingly weaker and/or shorter. Furthermore, this switch valve has a lip seal which, upon the start of the opening stroke of the valve disk, initially still remains "sticking" on the sealing seat and the seal tears off only when the holding action of the permanent magnet is overcome. The use of permanent magnets in switch valves is, however, rejected by most users since it is feared that particles of rust which are suspended in the water will be attracted and interfere with the operation. Furthermore, it has been found that the lip seal in combination with the protruding rim of the valve disk produces an additional push in the direction of opening, which must be compensated for by increased magnet and/or spring force.